1. Field of the Invention
The present invention relates to a conductive rubber roller for use in image forming apparatus such as electrophotographic copying machines, printers, and electrostatic recording apparatus. More particularly, the present invention relates to a transfer roller used in a transfer device for transferring to a recording medium or a transfer material, such as paper, transferable images constituted of toner images which are formed by making use of an image forming means, such as an electrophotographic process and/or an electrostatic recording process, and then held on an image carrying member such as an electrophotographic photosensitive member.
2. Description of the Related Art
In image forming apparatus such as electrophotographic copying machines and/or electrostatic recording apparatus, a contact charging system is mainly used which involves pressing a conductive rubber roller, to which a voltage is applied, against the surface of an electrophotographic photosensitive member for charging. The conductive rubber roller is used individually in each of steps, such as charging and transfer steps, and disposed around the electrophotographic photosensitive member drum, which is a main element for image formation.
In recent years, polar rubber such as acrylonitrile-butadiene rubber or epichlorohydrin rubber has been used as a rubber component for such a conductive rubber roller. It has been known that the polar rubber exhibits conductivity (ion conductivity) due to the presence of a polar group in its polymer molecule and it is suitable for use in the conductive rubber roller because of its small variation in the electrical resistance and small voltage dependence of the electrical resistance.
The above-mentioned conductive rubber roller is, in most cases, required to possess an elastic body layer having a volume resistivity of 2×109 Ω·cm or less. A vulcanized product has a volume resistivity of about 2×109 Ω·cm to 1×1010 Ω·cm when its rubber component is acrylonitrile-butadiene rubber alone, resulting in an insufficient conductivity.
Therefore, used is commonly an adjustment method for obtaining a desired volume resistivity by blending epichlorohydrin rubber, which is known to provide a vulcanized product having a volume resistivity of about 1×107 to 3×109 Ω·cm, to acrylonitrile-butadiene rubber (e.g., Japanese Patent No. 3656904).
Further, in recent years, a conductive rubber roller with a lower resistance has been demanded in order to achieve the formation of color images and provide high quality images. Hence, used is a method of using epichlorohydrin rubber alone or a method of adding to the rubber various ion conductive agents such as quaternary ammonium salts containing a perchlorate ion and a chloride ion (e.g., Japanese Patent Application Laid-Open No. 2002-132020).
Generally, however, in the case of a conductive rubber roller using such a rubber elastic body, its resistance value changes with temperature and humidity, which may cause a problem that the quality of images obtained varies depending on the usage environment. Moreover, because the acrylonitrile-butadiene rubber is inferior in ozone resistance, there may be another problem that sufficient electrification durability is not achieved. Moreover, in the above-mentioned method of adding various ion conductive agents such as quaternary ammonium salt, there is a possibility that the contamination by surface migration occurs and the resistance value changes with time due to electrification, and in consequence, a satisfactory electrification durability to cope with the requirements for high speed and long-life use may not be achieved.
As a method of solving the above-mentioned problems, a method of blending two kinds of carbon blacks of Ketjenblack and thermal black whose properties are different from each other in a blend of acrylonitrile-butadiene rubber and an ethylene-propylene-diene copolymer (EPDM) has been proposed (e.g., Japanese Patent No. 3401995).
Moreover, a method of adding a conductive filler such as carbon black to epichlorohydrin rubber has also been proposed (e.g., Japanese Patent Application Laid-Open No. 2005-043703).
A conductive roller using such a conductive rubber material allows a conductive rubber roller that is excellent in environmental dependence and electrification durability to be obtained. However, depending on the dibutyl phthalate (DBP) oil absorption and the nitrogen adsorption specific surface area (N2SA) of the carbon black to be used, the resistance value easily fluctuates according to the dispersion state of the carbon black in the rubber material, and the resistance value varies depending on the material lot. The resistance also varies locally in the conductive roller due to slight differences in the dispersion state of the carbon black. Moreover, there is a problem that it is difficult to obtain a conductive roller having a uniform and stable resistance value because the resistance value fluctuates due to slight deviations in conditions, such as the extruding pressure used in the extruding step. Further, a conductive roller using such a conductive rubber material also has a problem that a precise control device for the applied voltage is required so as to obtain a constant resistance value because the dependence of the resistance value on the applied voltage is high.